For high-temperature processes, the petrochemical industry requires materials which are temperature-resistant as well as corrosion-resistance and able to withstand, on one hand, the hot product gases and, on the other hand, also the hot combustion gases, for example, from steam crackers. Their tube coils are exposed on the outside to the oxidizing nitrogen-containing combustion gases having temperatures of 1100° C. and above, as well as in the interior to temperatures reaching approximately 900° C. and potentially also high-pressure of a carburizing and oxidizing atmosphere.
As a result, the nitrogen content of the tube material increases starting from the exterior tube surface and a scale layer is created in contact with the hot combustion gases.
The carburizing hydrocarbon atmosphere inside the tube carries the risk that carbon diffuses therefrom into the tube material, causing the carbides in the material to increase, forming from the existing carbide M23C9 with increasing carburization the carbon-rich carbide M7C6. Internal stress results from the volume increase of the carbides caused by the formation and conversion of carbide, and the strength and the ductility of the tube material are also reduced. In addition, a firmly adhering coke layer having a thickness of several millimeters is produced on the interior surface. Cyclic temperature stresses, for example caused by a shutdown of the plant, also cause the tubes to shrink onto the coke layer due to the different thermal expansion coefficients of the metallic tube and the coke layer. This causes large stresses in the tube which in turn cause cracks in the interior tube surface. An increasing amount of hydrocarbons can then enter the tube material through these cracks.
The U.S. Pat. No. 5,306,358 discloses a nickel chromium iron alloy which is weldable with the WIG process and has up to 0.5% carbon, 8 to 22% chromium, up to 36% iron, up to 8% manganese, silicon and niobium, up to 6% aluminum, up to 1% titanium, up to 0.3% zirconium, up to 40% cobalt, up to 20% molybdenum and tungsten as well as up to 0.1% yttrium, with the remainder being nickel.
The German patent 103 02 989 also describes a nickel chromium cast alloy suitable for tube coils of cracker and reformer furnaces with up to 0.8% carbon, 15 to 40% chromium, 0.5 to 13% iron, 1.5 to 7% aluminum, up to 0.2% silicon, up to 0.2% manganese, 0.1 to 2.5% niobium, up to 11% tungsten and molybdenum, up to 1.5% titanium, 0.1 to 0.4% zirconium, and 0.01 to 0.1% yttrium, with the remainder being nickel. This alloy has proven itself especially for the use as material for tubes; however, users still demand tube material with a prolonged life cycle.
The invention is therefore directed to a nickel chromium alloy with improved stability under conditions occurring, for example, during cracking and reforming of hydrocarbons.